Add closeBraceTokenFor and use it instead of BeginToken.

pkg/front_end/lib/src/fasta/parser/util.dart

Index: pkg/front_end/lib/src/fasta/parser/util.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/util.dart b/pkg/front_end/lib/src/fasta/parser/util.dart
index f5404ce56a4c112b7d3c689e7018747e923d177c..d392a266d7e179aeb796245e277ee164a7603957 100644
--- a/pkg/front_end/lib/src/fasta/parser/util.dart
+++ b/pkg/front_end/lib/src/fasta/parser/util.dart
@@ -6,7 +6,13 @@ library fasta.parser.util;
 
 import '../scanner.dart' show Token;
 
+import '../../scanner/token.dart' show BeginToken;
+
 /// Returns true if [token] is the symbol or keyword [value].
 bool optional(String value, Token token) {
   return identical(value, token.stringValue);
 }
+
+/// Returns the close brace, bracket, or parenthesis of [left]. For '<', it may
+/// return null.
+Token closeBraceTokenFor(BeginToken left) => left.endToken;